DETROIT - A former leader of the NFL's committee on concussions told Congress on Monday more study needs to be done on the effects that performance-enhancing drugs could have on the brains of football players.

Dr. Ira Casson was among the witnesses scheduled to testify at a House Judiciary Committee hearing at Wayne State University about brain injuries in football. Lawmakers had complained that Casson did not appear at the panel's hearing on the same topic in October.

He resigned as co-chairman of the NFL's committee on mild traumatic brain injury in November. Casson had served on the committee since its founding in 1994.

In written testimony Monday, Casson says that "there is not enough valid, reliable or objective scientific evidence at present to determine whether or not repeat head impacts in professional football result in long term brain damage."

He also lays out reasons why recent studies, including some funded by the NFL, examining the possible connection between head injuries in football players and brain disease later in life have been flawed.

"Some have suggested that scientific evidence regarding the question at hand is conclusive and that there is no need for further research," Casson writes. "I strongly disagree with that position."

Among the additional scientific research Casson recommends is a study of "the longterm effects of use of performance enhancing drugs on brain pathology." He also says the NFL should "continue efforts to prevent concussions by improving safety equipment and making appropriate rule changes."

NFL commissioner Roger Goodell was not present at Monday's hearing. At the October hearing, Goodell was grilled by lawmakers about his league's concussion policies, and Rep. Linda Sanchez, D-Calif., said the NFL's resistance to accepting a link between multiple head injuries in NFL players and brain disorders such as dementia and Alzheimer's reminded her of tobacco companies denying a link between smoking and disease.

Since then, the league has instituted stricter return-to-play guidelines for players showing concussion symptoms; required each team to enlist an independent neurologist as an adviser; entered into a partnership with Boston University brain researchers who have been critical of the league's stance on concussions; and conducted tests on helmets.

Goodell also accepted the resignations of Casson and the league's concussion committee's other co-chairman. Although a successor has not been named, the NFL has five finalists and plans to pick a new head of its committee by the end of this month.

In addition, the NFL announced Monday it has added two new members to that committee: former player Merrill Hoge and University of Michigan neurologist Jeffrey Kutcher.